1. Field of the Invention
The present invention relates to a method for making a flexographic printing master by inkjet printing and an imaging apparatus for performing the method.
2. Description of the Related Art
Flexography is commonly used for high-volume runs of printing on a variety of supports such as paper, paperboard stock, corrugated board, films, foils and laminates. Packaging foils and grocery bags are prominent examples.
Today flexographic printing forms are made by both analogue imaging techniques such as a UV exposure through a film mask, e.g. EP 1594005 (DUPONT), and digital imaging techniques which include:                direct laser engraving on flexographic printing form precursors, e.g. US 2004/0259022 (BASF);        UV exposure through a LAMS mask, e.g. U.S. Pat. No. 6,521,390 (BASF) and U.S. Pat. No. 7,226,709 (KODAK), wherein LAMS means Laser Ablative Mask System;        Mask-less direct UV or violet exposure by laser or LED, e.g. U.S. Pat. No. 6,806,018 (MACDERMID); and        Inkjet printing e.g. EP 1428666 A (AGFA), US 2004/0131778 A (AGFA) and US 2006/0055761 (AGFA).        
EP 1428666 A (AGFA) discloses a method for making a flexographic printing form by jetting subsequent layers of a UV-curable liquid, having elastomeric properties after being cured. Before jetting the following layer each previous layer is immobilized by a UV curing step. This “layer after layer” recording technique allows the gradual building up of a flexographic printing master wherein the relief can be accurately controlled. Use can be made of different curable liquids or immobilisation steps to obtain different layer characteristics.
Advantages of such a method for preparing a flexographic printing master are the absence of any processing steps and the consumption of no more material as necessary to form a suitable relief image, i.e. the removal of non printing areas is no longer required.
However, several difficulties in controlling the printing surface of the reliefs formed via ink-jet printing can occur. The printing surface of a flexographic printing relief is important because of its role in ink reception from the anilox roller and subsequent transfer to the substrate. Flexography prints with a “kiss impression”, i.e. the least possible squeeze between printing form and substrate.
A flexographic printing form having a very smooth surface often produces printing results having relatively low ink densities in the centre of solid areas while the edges of these solids result in a larger ink density. Such problems can be resolved by incorporating a surface roughness during the flexographic printing master fabrication, e.g. by incorporation matting agents.
It must be clear that the topographic result of different layers of UV-curable liquids subsequently jetted on each other depends upon the drop volume and the spreading properties of the ink used. Especially in solid image areas and in the coarsest printing dots, the surface of the printed relief will be quite rough and uneven. A larger ink drop size in combination with bad spreading properties will intensify this effect. Also the time between jetting and curing has an influence on the surface evenness.
As the surface finish of the printed relief is very important in order to provide excellent printing results, it should be clear that a high surface unevenness can lead to an important loss in Dmax of the printed image and/or a lot of print mottle.
In creating small printing dots on the flexographic printing master, a dot profile will generally have a rounded top, resulting in a smaller printed dot size with the flexographic printing master. Another phenomenon which occurs in creating very small printing dots is the flow down of curable liquid when a next ink drop is deposited on top of such a small printing dot. As a result the smaller printing dots will be less high than larger printing dots or than the solid image areas and the image information of these small printing dots is not printed upon a substrate.
It is at present not clear which adaptations in image processing should be made to obtain an inkjet printing mode capable of resolving all of the above problems. A need exists for making flexographic printing masters by inkjet printing which exhibit high printing quality.